Can't help you with fabricating a case or flat mounting on a '66, but I used a tach from a Chevy Cavalier Z24 with V6.
I have a 292 with HEI. Fortunately the '67-'72 already has a location for a tach and it wasn't to difficult to modify.
Aftermarket is probably your best bet for a '66.
The Cavalier also had a great small diameter, FAT rim steering wheel. With power steering I don't need that wagon wheel size that was original. The original GMC horn button fit the Cavalier wheel with only a small knife trim on a tab.
